Week 9: heaps, priority queues
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We finished to review trees with the introduction of heaps, that are a specific kind of binary tree.
Actually, when we speak about heaps, they are either min or max heaps. 

A min heap is a tree satisfying the following properties:
- it is a binary tree;
- it is a complete tree;
- the root is <= than both its children;
- the left sub-tree and the right sub-tree are also min heaps.

We discussed how to build a heap, and also how to remove the root of the heap and to re-heap it.

Then we discussed how a heap can be seen as a priority queue. Since we had already discussed about priority queues earlier when presenting queues, the concept was not new.